1. Field of the Invention
The present invention relates to a transformer with an additional integrated inductor, and to a switched-mode power supply comprising such a transformer.
Switched-mode power supplies require at least one inductive component and one capacitive component for the resonant circuit. The inductive component is usually a coil (inductor) which along with a capacitor is connected to the primary or secondary winding of a transformer.
2. Description of the Related Art
Such a switched-mode power supply with a resonant construction is known from U.S. Pat. No. 4,692,851. Here, the primary winding is connected in series with an inductor, whereas, on the secondary side, the main secondary winding has a parallel capacitance. In this way, the inductance of the inductor alone with the capacitance of the capacitor provide the necessary resonant oscillations.
It is an object of the present invention to provide a transformer with an integrated stray inductance so that a separate inductive component can be dispensed with and a simplification of the design of resonant switched-mode power supplies is possible.
This object as achieved, in accordance with the invention, in that a transformer is provided with a first core which has at least a primary winding, a second core which has at least a secondary winding, and a third core, each core being a separate component.
Through the structural separation of the individual cores, it is possible to optimize each of these in relation to the main flux or the stray flux. Nevertheless, standard components can be used, because each core only has one section. This reduces the manufacturing costs considerably. Since the main flux is dependent upon the so-called main permeance and the stray flux on the so-called stray permeance, these permeances must be selected accordingly. The permeances are, in turn, dependent upon the respective air gap between the individual cores, the respective air gap having a certain cross-sectional area and a certain length. The cores can, however, be bonded to each other when only little permeance is necessary.
Embodiments of the transformer as described above, in which the first core is separated from the second core by at least an air gap, and the second core is separated from the third core by at least a further air gap, and in which the air gap and the further air gap are designed so that the first core essentially carries a main flux, the third core essentially carries a stray flux, and the second core essentially carries the difference between the main flux and the stray flux, offer the advantage that the three cores can be optimally matched to the flux that they carry. Thus, both the main flux and the stray flux may be determined individually. In addition, the stray permeance and the main permeance can be determined via the air gap between the individual cores.
A further embodiment of the transformer as described above, in which the stray flux through the third core has the effect of an inductor integrated with the transformer, this inductor effectively being connected in series with the secondary winding, offers the major advantage that an additional coil (inductor) in series with either the primary winding or the secondary winding is not necessary since resonant switched-mode power supplies require such a coil, their structure is thus simplified since one component can be spared.
An embodiment of the transformer of the subject invention, as described above, in which the first core, the second core and the third core each have a standardized section, allows a particularly low-cost design of a transformer in accordance with the invention. The manufacturer of magnet cores for transformers offers all standard sections. Magnet cores with other sections are considerably more expensive which, in mass-production, is a factor that cannot be underestimated. It is therefore sensible to use standard components for the three magnet cores of different dimensions.
With the embodiment of the invention, in which a switched-mode power supply with a transformer provided with a first core having at least a primary winding, a second core having at least a secondary winding, and a third core, each core being a separate component, the advantage of the variable stray inductance of a transformer in a switched-mode power supply is used. This is preferably a resonant switched-mode power supply.